Method and bath for electroplating rhenium



United States Patent 3,285,839 METHOD AND BATH FOR ELECTROPLATING RHENIUM Eldridge K. Camp, WatertoWn, Conn., assignor to Amer:-

can Chemical & Refining Company, Incorporated, Waterbury, Conn, a corporation of Connecticut No Drawing. Filed Dec. 16, 1963, Ser. No. 330,569 14 Claims. (Cl. 204-47) The present invention relates to the electroplating of rhenium and, more particularly, to a novel bath and method for electroplating rhenium metal from aqueous electrolytes.

Rhenium has been considered highly attractive as a plated surface material because of its high hardness, high melting temperature, high density and reliability for electrical conductor applications since its oxide is conductive. Various methods and baths have been proposed for the electroplating of rhenium including those proposed in United States Patent Number 2,138,573 to Fink et al. and United States Patent Number 2,616,840 to Levi. References in the literature include G. S. Root and J. G. Beach, Electroplating of Rhenium, Rhenium, a collection of rhenium papers presented at the Electrochemical Society Meeting of May 1960, and edited by B. W. Gonser and published by Elsevier, 1962, pages 181188; and L. E. Netherton and M. L. Holt, Electrodeposition of Rhenium From Aqueous Solutions, Transactions Electrochemical Society, volume 95, June 1949, pages 324- 328.

Although rhenium has such highly desirable properties, it has not enjoyed the degree of usage which would be anticipated therefrom because of a tendency for the asplated deposit to corrode or oxidize in air and particularly rapidly on exposure to moisture. The aforementioned Levi patent discloses a method for eliminating internal stresses in relatively thick deposits of the metal by subjecting layers of the plated metal to a treatment in a reducing atmosphere at a temperature of about 1000 centigrade and above. This treatment has proven satisfactory for improving the plated deposits but has increased the cost and the difiiculty in obtaining electroplated rhenium deposits.

It is the aim of the present invention to provide a novel method for electroplating rhenium which is characterized by relative ease and low cost and which produces a bright metallic deposit having high resistance to corrosion or oxidation in moist atmospheres over extended periods of time. A related aim is to provide a simple and easily formulated bath for such a method which may be readily controlled and operated.

It has now been found that the foregoing and related aims may be readily attained in a method wherein the article to be plated is immersed in a bath containing essen: tially soluble rhenium ion, phosphate ion and a noninterfering cation at a pH of about 4.0 to 6.8, and preferably about 4.8 to 6.5. The temperature of the bath should be maintained at above 50 centigrade and preferably at about 57 to 66 centigrade. A temperature of 90 centigrade may generally be regarded as the practical upper limit for most operations, although temperatures up to boiling may be employed albeit with a tendency for volatilization and decomposition of the components.

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The rhenium ion is plated onto the cathode by use of an inert anode and a current density of about 15 to amperes per square decimeter.

The non-interfering cation may be the ammonium ion or the alkali metal ions. The ammonium ion has proven highly advantageous and is preferred because of apparently enhanced operation and quality of deposit. Although the theory of operation for its enhanced effect is not known, it is believed to produce a beneficial complex in the bath. The potassium ion has proven least desirable since the deposits using potassium as a cation have exhibited the least corrosion resistance. Sodium is the preferred alkali metal ion. The amount of the noninterfering cation will be determined by the soluble rhenium compound added, i.e. Whether perrhenic acid is utilized or the ammonium or alkali metal perrhenate, and by the amount of and particular phosphate salt added, and by the amount of any ammonium or alkali metal base that may be added to adjust the pH.

The soluble rhenium ion should be above about 4 grams per liter (calculated as metallic rhenium) in the initial formulation, although it appears possible to plate until the bath is totally or nearly exhausted. Although the rhenium ion may be added to saturation or even in excess provided that the precipitate does not interfere with the conditions of operation, the preferred baths utilize a concentration of about 10 to 40 grams per liter. The concentration in the bath during operation may be readily determined colorimetrically according to the method of E. A. Malouf and M. G. White, Analytical Chemistry, volume 23, pages 497-499 (1951).

Although the alkali metal perrhenates may be employed in formulating the bath, the preferred soluble rhenium compounds are ammonium perrhenate (NH ReO perrhenic acid (HReO perrhenic heptoxide (R3207) and rhenium phosphate so as to minimize unessential ions and permit use of the preferred ammonium ion without alkali metal ions.

Although alkali metal phosphates and acid phosphates may be employed in formulating the bath, the preferred phosphates are ammonium dihydrogen phosphate and diammonium hydrogen phosphate because of their introduction of the preferred ammonium ion and production of a pH near the desired range, thus minimizing the problem of pH adjustment.

Since the pH of the initial formulation may deviate from the prescribed range depending upon the soluble phosphates and rhenium compounds employed, the pH should be adjusted by the base of a non-interfering cation, preferably ammonium hydroxide, or by phosphoric acid in the event the bath should prove too alkaline.

The bath is preferably subjected to stirring during the plating operation to remove possible gas bubbles from the cathode and maintain current efficiency at low current density. Although mechanical stirring may be employed, ultrasonic stirring has shown enhancement of the corrosion resistance of the plated deposit upon exposure to ambient atmospheres.

Various anodes substantially inert to the bath and plating conditions may be employed including platinum and platinum-clad titanium anodes. Graphite or carbon anodes may be employed if the bath is subjected to continuous filtration so as to remove any carbon particles. Even if inert metal anodes are employed, continuous filanode ratios being preferable.

tration is desirable to minimize possible interference from contaminant particles.

The concentration of phosphate ion does not appear to be critical although it should be well in excess of the molar equivalent of the rhenium in the formulation. Generally, about 25 to 250 grams per liter of phosphate ion should be employed and preferably about 50 to 150 grams per liter.

The thickness of the rhenium plate may vary dependent upon the intended application. For example, use in cathode emitters has required a deposit of about 5 to microinches; use as a barrier material has required a deposit about 0.1 to 0.3 mil; and use as wear-resistant surface has required a deposit of about 0.3 to 0.6 mil. Generally, deposits of up to 0.5 mil have been highly satisfactory in the as-plated condition. Deposits of 2 mils and above have been obtained but have shown evidence of high brittleness and severe compressive stress,

particularly when obtained at high current density, although a high degree of corrosion resistance has been apparent. Thick plates are desirably produced at lower current density as exemplified by the fact that a deposit of 1.0 mil at low current density evidenced no cracks at a magnification of 400x.

The hardness of the plated metal has been found to increase With increase in current density. Generally, it is desirable to use lower current density to minimize the apparent compressive stress created in the metal deposit since highly desirable hardness will nevertheless be obtained although some application may justify use of the higher current densities for the greater hardness and may be coupled with an annealing treatment of the type disclosed in the aforementioned Levi patent. Stress relievers such as saccharine and sulfur-bearing agents have been incorporated in the formulation with some evi- "dence of beneficial effect.

The introduction of interfering materials should be avoided although it may be beneficial to incorporate stress relievers as indicated above, brighteners and wetting agents. Various intentional impurities may be included for special effects such as gold to obtain a pinkish appearance, copper to obtain a dull, satin finish and platinum to obtain a dark but extremely highly corrosion-resistant deposit.

Rhenium may be plated directly onto various conductive materials including graphite, platinum, rhodium,

molybdenum, tungsten, tantalum, gold, silver, iron, stainless steel, copper, nickel, chromium and brass. In plating some materials such as graphite, tungsten, molybdenum and tantalum, a strike of nickel or other metal is beneficial although other preparatory surface treatments may be employed such as mechanical cleaning or abrasion, (grit blasting). Since the bath is readily contaminatedby copper, strikes or flashes of nickel, gold or other metal are desirably employed when plating copper or brass articles. Rhenium may be overplated readily with various metals if desired including gold, nickel and copper.

Although the voltage will vary with the operating conditions, it will be in the range of about 2 to 18 volts and preferably about 6 to 12 volts. The anode to cathode ratio may vary from about 0.5 :1 to 4:1 with the greater As previously indicated, the current density may vary in the range of about 15 to 75 amperes per squaredecimeter, and preferably about 27 to 40 amperes per square decimeter. Above about 45 amperes, the deposit tends to develop rapidly and be more subject to compressive stress and have less corrosion resistance.

The following is a specific example of a preferred bath formulation for the present invention:

Ammonium perrhenate, grams/liter 10 'Ammoniurn dihydrogen, phosphate, grams/ liter 200 Ammonium hydroxide (28% by wt.), milliliters 8O pH6.0.

TABLE ONE A ccelerated test Corrosion Resistance (Percent Metallic Surface) Panel pH 50 Days 64 Days TABLE TWO Non-accelerated test 1 Corrosion Resistance (13 Months), Percent Metallic Surface In this test, the panels are merely subjected to normal ambient atmospheres and temperatures.

2 Exhibited definite dull blue cast or tarnish.

3 Exhibited slight dull cast.

4 Very bright with no cast.

Indicative of the effect of the several components and of the effect of increasing concentration of the ammonium and phosphate ions is the data in Table Three. The tests were made using nickel test panels 1 centimeter X 4 centimeters x 5 mils in an aqueous bath at an average temperature of 60 'centigrade 8 centigrade with a platium basket anode for a ten-minute period.

TABLE THREE Efiect of component ions Concentration, Mols Deposit Specimen pH Weight,

mg. HReOi 1131 04 NILOH NH4HgP04 0. 01 0. 8 l7. 5 O. 01 9. 0 2. 3 0. 01 0. 01 5. 6 8. 8 O. 01 0. 01 5. 6 9. 3 0.01 0. 01 5. 6 ll. 6 0. 01 0. 01 5. 7 9. 0 0. 01 0. 01 5. 7 8. 7 0. 01 0. 01 5. 4 8. 0 0. 01 0. 01 5. 4 7. 5 0. 01 0. 01 5. 4 7. 7 0. 01 0. 01 0. 1412 0.3372 5. 4 6. 1

*These specimens were run for five minutes only and deposit is based upon double amount obtained for five minute period.

amperes per square decimeter and the voltage generally decreased for each addition from 29 volts for Specimen 3 to 10 volts for Specimen 11.

Specimen 1 had a dirty dark deposit and Specimen 2 had a smutty black deposit. The remaining specimens produced in accordance with the present invention had a bright finish which generally increased further in brightness to that of Specimen 11. Specimens 3-7 had relatively little resistance to finger staining, Specimens 8-10 had moderate resistance to finger staining and Specimen 11 had very good resistance to finger staining.

Exemplary of the efiicacy of the present invention are the following specific examples.

Example One A bath was prepared in accordance with the preferred formulation above and placed in a :glass tank. The bath was heated to a temperature of about 60 centigrade and a platinum-clad tantalum anode was employed and calculated to have an anode-to-cathode ratio of about 2:1. The cathode in this instance was a spiral strip utilized in the Brenner-Senderoff Contractometer. The current density was about 30 amperes per square decimeter, and the voltage was about 12. After plating for about 60 minutes, during which the bath was stirred e-lectromagnetically, the strip was removed and rinsed.

The resulting deposit was bright and uniform and was found to be 42 microinches in thickness (calculated by weight gain). The deposit Was tested with a Brenner- Senderolf Contractometer and found to have a maximum stress of 15,000 psi. (compressive).

Example Two A bath was prepared containing 16 grams per liter of potassium perrhenate and 200 grams per liter of ammonium dihydrogen phosphate. The pH of the bath was 4.5 and the bath was heated to a temperature of 80 centigrade. A platinum-clad tantalum anode was employed, and a brass panel 0.5 inch x 2 inches x 0.003 inch having a thin gold strike was employed as the cathode. The panel was plated at a current density of 17 amperes per square decimeter.

Upon removal from the bath and rinsing, it was observed that the panel had a bright, uniform metallic finish. The panel was tested for microhardness according to the manner reported in ASTM E 92-57 using a Bergsman Microhardness Tester. The microhardness was determined to be 802 kilograms per square millimeter V.H.N. at a 20 gram load.

Example Three An aqueous bath was prepared containing 37.5 grams per liter of perrhenic acid, 70 grams per liter of disodium hydrogen phosphate and 190 grams per liter of sodium dihydrogen phosphate. The bath had a pH of about 5.9 and was heated to a temperature of 57 centigrade. A platinum-clad tantalum anode was employed and a nickel panel 1 centimeter x 4 centimeters x mils was used as the cathode with an anode-to-cathode ratio of about 2:1. The current density was 21 amperes per square decimeter.

Upon removal from the plating bath and rinsing, the deposit was observed to have a very attractive, bright finish that resembled stainless steel.

Thus, it can be seen from the foregoing specific examples and the detailed specification that the present invention provides a novel method and bath for electroplating rhenium which is characterized by ease and low cost and which produces a bright metallic deposit having high resistance to corrosion in moist atmospheres over extended periods of time. The deposit has high hardness and may be produced relatively free from stress cracking in relatively high thicknesses even in the as-plated condition. The method thus permits plating of parts which are not feasibly annealed or otherwise treated to minimize stress cracking.

Having thus described the invention, I claim:

1. In the method of electroplating rhenium, the steps comprising immersing a conductive article in an aqueous bath consisting essentially of soluble rhenium ion, phosphate ion and a non-interfering cation selected from the group consisting of ammonium ions, alkali metal ions and the combination thereof, said bath having a pH of about 4.0 to 6.8 and a temperature above about 50 centigrade; and passing current through said bath at a current density of about 15 to 75 amperes per square decimeter of surface area of said article.

2. The method of claim 1 wherein said cation is ammonium ion.

3. The method of claim 1 wherein said pH is about 4.8 to 6.5.

4. The method of claim 1 wherein said temperature is about 57 to 66 centigrade.

5. The method of claim 1 wherein said current density is about 27 to 40 amperes per square decimeter.

6. In the method of electroplating rhenium, the steps comprising immersing a conductive article in an aqueous bath consisting essentially of soluble rhenium ion in an amount of above about 4 grams per liter to saturation (calculated as metallic rhenium), phosphate ion in excess of the molar equivalent of rhenium ion, and a non-interfering cation selected from the group consisting of ammonium ion, alkali metal ion, and the combination thereof, in excess of the molar equivalent of rhenium ion, said bath having a pH of about 4.0 to 6.8 and a temperature of about 50 to centigrade; and passing current through said bath at a current density of about 15 to 75 amperes per square decimeter of surface area of said article.

7. The method of claim 6 wherein said cation is ammonium ion.

8. The method of claim 6 wherein said pH is about 4.8 to 6.!5 and said temperature is about 57 to 66" centigrade.

9. In the method of electroplating rhenium, the steps comprising immersing a conductive article in an aqueous bath consisting essentially of soluble rhenium ion in an amount of above about 4 grams per liter to saturation (calculated as metallic rhenium), phosphate ion in excess of the molar equivalent of rhenium ion, and ammonium ion in excess of the molar equivalent of rhenium ion, said bath having a pH of about 4.8 to 6.5 and a temperature of about 57 to 66 centigrade; and passing current through said bath at a current density of about 15 to 75 amperes per square decimeter of surface area of said article.

10. An aqueous bath for the electroplating of rhenium consisting essentially of soluble rhenium ion, phosphate ion and a non-interfering cation selected from the group consisting of ammonium ion, alkali metal ion and the combination thereof, said bath having a pH of 4.0 to 6.8.

11. The aqeuous bath of claim 10 wherein said bath has a pH of about 4.8 to 6.5.

v 12. The aqueous bath of claim 10 wherein said cation is ammonium 1011.

13. An aqueous bath for the electroplating of rhenium consisting essentially of soluble rhenium ion in an amount of above about 4 grams per liter (calculated as metallic rhenium), phosphate ion in excess of the molar equivalent of rhenium ion, and non-interfering cation selected from the group consisting of ammonium ion, alkali metal ion and the combination thereof, in an amount in excess of the molar equivalent of rhenium ion, said bath having a pH of about 4.0 to 6.8.

14. The bath of claim 13 wherein said cation is ammonium ion and said pH is about 4.8 to 6.5.

References Cited by the Examiner UNITED STATES PATENTS 2,616,840 11/1952 Levi 204-47 X 2,788,786 1/1957 Pearlman et al 204-47 X (Other references on following page) 7 OTHER REFERENCES Chemical Abstracts, vol. 53, N0. 7, col. 6202 and 6203, April 3, 1961.

L. E. Netherton, et al., Electrodeposition of Rhenium- Cobalt and Rhenium-Iron Alloys, Journal of the Electrochemical Society, vol. 99, p. 44, 1952.

L. E. Netherton, et a1., Electrodeposition of Rhenium 'from'Aqueous Solutions, Transactions of the Electrochemical Society, vol. 95, pp. 324-328, 1949.

L. E. Netherton, et 211., Electrodeposition of Rhenium-Nickel Alloys, Journal of the Electrochemical Society, vol. 98, pp. 106-109, 1951.

JOHN H. MACK, Primary Examiner. G. KAPLAN, Assistant Examiner. 

1. IN THE METHOD OF ELECTROPLATING RHENIUM, THE STEPS COMPRISING IMMERSING A CONDUCTIVE ARTICLE IN AN AQUEOUS BATH CONSISTING ESSENTIALLY OF SOLUBLE RHENIUM ION, PHOSPHATE ION AND A NON-INTERFERING CATION SELECTED FROM THE GROUP CONSISTING OF AMMONIUM IONS, ALKALI METAL IONS AND THE COMBINATION THEREOF, SAID BATH HAVING A PH OF ABOUT 4.0 TO 6.8 AND A TEMPERATURE ABOVE ABOUT 50*CENTIGRADE AND PASSING CURRENT THROUGH SAID BATH AT A CURRENT DENSITY OF ABOUT 15 TO 75 AMPERES PER SQUARE DECIMETER OF SURFACE AREA OF SAID ARTICLE. 